<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div></div>


<script>
   function f(x) {
        //如果给元素添加了id  会自动添加下面两行代码
        // let i1 = document.querySelector("#i1");
        // let i2 = document.querySelector("#i2");
        let d = document.querySelector("div");
        if (isNaN(i1.value)||isNaN(i2.value)){
            d.innerText="输入错误！";
            return;  //结束方法  不在往下执行了
        }
        switch (x) {
            case 1:
                d.innerText = i1.value*1+i2.value*1;
                break;
            case 2:
                d.innerText = i1.value-i2.value;
                break;
            case 3:
                d.innerText = i1.value*i2.value;
                break;
            case 4:
                d.innerText = i1.value/i2.value;
                break;
        }
   }
</script>
</body>
</html>